Despite many advances in the field of cancer diagnosis and treatment, a reliable method of identifying and treating cancer cells while sparing non-cancerous cells has been elusive. One of the limitations is the heterogeneity of human cancers. It has therefore been problematic to rely on any single tumor biomarker even for one type of cancer. Detection of tumor acidity may be an alternative strategy for targeting tumor cells. A pH-sensitive polypeptide with a predominantly hydrophobic sequence long enough to span a membrane lipid bilayer as a transmembrane helix and two flanking sequences (FS) has been described (WO 2006/078816 A2). Selective and efficient targeting and delivery of therapeutic agents to tumor cells remains a challenge.